Rubber or impact modified (or resistant) thermoplastics such as high impact polystyrene (HIPS), acrylonitrile-butadiene-styrene plastic (ABS) and methyl methacrylate-butadiene-styrene plastics (MBS) have been known for a significant period of time. These rubber modified polymers comprise a small amount, typically from about 3 to 15 weight % of a rubbery substrate polymer onto which is grafted at least a portion of a superstrate polymer. The polymers are useful in a wide range of applications. However, the polymers tend to be subject to a significant loss of physical properties when subjected to long chain oils and/or fats. This is known as environmental stress cracking (ESC). The problem may exhibit itself first as a hazing of the plastic and on prolonged exposure to such oils or fats the plastics may crack or even break into several pieces. Unfortunately, a fairly significant market for thermoplastics is in housewares which are subject to chemicals which tend to cause ESC such as cleaners and in some cases fatty or oily food. To date, the thermoplastic having the best ESCR that Applicants are aware of, is Chevron's HIPS grade 6755. After analysis of the Chevron product, Applicants have concluded that the product contains from about 2 to 3 weight % of polybutene and has a dispersed rubbery phase having a volume average particle diameter of between 4 and 4.5 microns.
Applicants are also aware of some trade literature, also by Chevron, which suggests that polybutene may be useful to enhance the ESCR of some plastics.
Applicants have been unable, after a search of various computerized patent and literature databases to find any clear references disclosing the combination of large particle size of the rubbery substrate and the use of polybutene.
The present invention seeks to provide a rubber modified thermoplastic having enhanced ESCR.